The increase in CO2 concentration in the atmosphere and the greenhouse effect caused by CO2 is of great concern, and has caused a change towards renewable energy resources and an environmental pressure towards stopping or at least reducing the use of fossil fuels. However, due to both the low cost of carbonaceous fuels and the steady growing demand for energy, and especially energy in the form of electricity for further development in the less industrial parts of the world, carbonaceous fuels will be important as energy sources for decades to come. CO2 capture and storage/deposing of CO2 to prevent the release of CO2 into the atmosphere and hence mitigate the increasing CO2 concentration, has therefore been proposed. Many projects on CO2 capture have been suggested, but only a few have been developed from the idea or drawing stage to actual projects, due to both the investment cost and running cost of such plants, and the lack of political support.
Most suggestions for CO2 capture plants are based on post combustion capture of CO2 from the exhaust gases from an industry plant, such as a power plant based on carbonaceous fuels. Typically, CO2 is captured from the exhaust gas by bringing the exhaust gas in intimate contact with a CO2 absorbent, most commonly an aqueous solution of an amine absorbent, a carbonate absorbent, etc., normally at a pressure close to atmospheric pressure or slightly higher.
WO 2004001301 A2, i.a. granted as U.S. Pat. No. 7,328,581, to Sargas AS, now assigned to CO2 CapSol AS, relates to a method and plant for generation of power by combustion of carbonaceous fuel under elevated pressure of typically 8 to 20 bar, capturing CO2 at the same pressure by means of an aqueous potassium carbonate solution, before the CO2 lean exhaust gas is expanded and released into the surroundings. An important feature of U.S. Pat. No. 7,328,581 is that CO2 is captured at substantially the same pressure as the combustion pressure for the carbonaceous fuel. CO2 capture under elevated pressure, as in U.S. Pat. No. 7,328,581, provides CO2 partial pressure, which is substantially higher than if the CO2 capture was to be performed at atmospheric pressure. It also reduces the total gas volume. The speed of reaction and system equilibria are highly dependent on the partial pressure of CO2 in the gas phase. Higher partial pressure of CO2 greatly improves the capture rate and reduces the absorbent regeneration energy. Furthermore, reduced gas volume greatly facilitates the design and construction of CO2 capture plants, in particular the CO2 absorber.
There are two classes of CO2 absorbents. These are inorganic, mainly aqueous potassium carbonate, and organic, mainly aqueous solutions of amines, mixtures of amines, or amino acids. Advantages with inorganic absorbent over organic absorbents are potentially decisive for very large scale CO2 capture. This includes stability in the presence of oxygen, low cost, availability in the significant quantities required for large scale CO2 capture and, if operated at elevated pressure, low specific regeneration energies. In contrast, organic absorbents are unstable in the presence of oxygen, in particular at high pressure where the oxygen partial pressure is higher, forming toxic and carcinogenic degradation products that will be released into the atmosphere together with the CO2 depleted exhaust gas. They have high cost, and limited availability since production is multifaceted and includes the production of ammonia, a process which emits CO2. Specific regeneration energy is high in particular if operated at atmospheric pressure. This is important since one of the principal challenges with CO2 capture is parasitic energy consumption.
U.S. Pat. No. 7,328,581 requires combustion at elevated pressure, as indicated above, and intimate integration of the combustion and power generation part and the CO2 capture part of such a plant. Accordingly, U.S. Pat. No. 7,328,581 is most suitable for new construction of power plants, and is normally not suitable as a solution for CO2 capture for existing plants.
WO 2013166301 A1, to Mecs, Inc., relates to a regenerative recovery of contaminants, exemplified by sulphur dioxide, from effluent gases. Nothing is mentioned about the pressure of the gas to be cleaned, or of recovery of heat energy in the incoming gas. The absorbent for removal of the contaminants, and heat of reactions for regeneration of the absorbent, are regenerated by different features to increase the energy efficiency of the process.
WO2007075466 A2, to Fluor Technologies Corporation, describes an integrated compressor/stripper configuration and methods, there regenerated absorbent is flashed and separated in a liquid phase that is recycled back to an absorber, and a gaseous phase that is compressed and introduced as stripper steam into the stripper.
WO 0048709, to Norsk Hydro ASA, relates to a method for capturing CO2, from an exhaust gas of a gas turbine based power plant, where expanded and cooled exhaust gas from the gas turbine power plant is re-compressed to a pressure of 5 to 30 bar, typically 7 to 20 bar, and cooled before the compressed gas is introduced into an absorber and brought in contact with an amine absorbent in an absorber of a CO2 capture plant. The CO2 depleted exhaust gas leaving the absorber is preferably re-heated, to increase the power to be generated by the expanding gas, and to increase the overall efficiency of the total plant. Such an integration is vital to be able to build such a plant so that it is sufficiently energy efficient, i.e. that the energy output of the plant in the form of useful energy, such as electrical power, is sufficiently high compared to the total energy of combustion in the gas turbine. Accordingly, it is highly unlikely that this concept can be built to be economically viable.
The object of the present invention is to provide a post combustion CO2 capture unit that is sufficiently energy efficient to be economically viable, allowing the CO2 capture unit to be added to an existing CO2 source plant, such as a power plant fired with carbonaceous fuel, without having to make internal rebuild of the CO2 source plant. Use of only electricity as its energy source is one of the keys to obtain this.